Synthesis of 1-phenylazo-2-naphtho 1-phenylazo-2-naphtholl
Jose Sandino A. Bandonil Institute of Chemistry, University of the Philippines, Diliman, Quezon City 30 October 2014 13 November 2014
Abstract 1-phenylazo-2-naphthol, known by the common name Sudan I, is a red carcinogenic dye previously used in colouring foodstuffs, plastics, and others; it is classified as an azo compound, a group characterised by the linkage of two arenes by two double-bonded nitrogen atoms. These compounds may be produced through a diazo coupling reaction between arenediazonium ions and highly reactive compounds; the arenediazonium ion may be prepared through the diazotisation of an amine. In the study, aniline was converted to phenyldiazonium chloride in the presence of sodium nitrite and strong acid, and subsequently reacted with phenolic compound β -naphthol to produce Sudan I dye. The melting point of the product was measured to obtain its qualitative purity. 2.97 grams of the dye was produced, with a melting point between 129°C -132 °C. I. Introduction 1-phenylazo-2-naphthol, also known by the common name Sudan I (Figure 1), is a red dye used in the colouring of several materials, including hydrocarbon solvents, oils, waxes, plastics, and many others; it was also used as a food colouring. However, it has been classified as a potential human carcinogen, due to its tendency to cause tumours in the liver and urinary bladder of rats, mice and rabbits (Stiborova et. al., 2002). Sudan I is classified as an azo compound.
Azo compounds are a group of compounds characterised by two arenes linked by two nitrogen atoms in a double bond. These compounds are notable for their use as textile dyes, due to their intense colouration. This is caused by the extended conjugated π electron system, which allows the absorption of light in the visible region of the electromagnetic spectrum. Azo compounds are formed through the diazo-coupling reaction, an electrophilic aromatic substitution reactions between an arenediazonium ion and a highly reactive compound. (McMurry, 2008).
Figure 1. 1-phenylazo-2-naphthol. Notice the double bonded N's linking the two arenes. The quality of the dyes formed from the diazo coupling reaction are judged by two qualities: its fastness and its levelness. Levelness is the uniformity of the dye upon its application on the surface of the fabric, while fastness is the ability of the dye to stick to maintain its colour.
Dyeing is the process of colouring objects, usually textiles, with a designated colour through the application of a dye on the surface to be coloured. The process of dyeing depends on the ability of the dye to gain access to the interior of the fibre, and its subsequent ability to stay fixed in its substrate. Several techniques are available in the application of dyes, including pigment dyeing, ingrain dyeing, and direct dyeing (Marshall Cavendish Corp.), among which ingrain and direct dyeing are the primary ones. Direct dyeing is the process involved for the colouring of a fibre with several polar sites through a polar dye; the process simply involves the immersion of the fibre in a single solution. On the other hand, ingrain dyeing, also known as developed dyeing, involves the soaking of the fibre on an alkaline
phenol solution, subsequently followed by immersion in a diazonium salt solution, letting the diazo coupling reaction forming the dye take place in the fibres (Seager & Slabaugh, 2013). II. Methodology To form the dye, the reagents phenyldiazonium chloride (C6H5N2Cl) and β-naphthol (C 10H7OH) for the diazo coupling reaction was prepared.
To prepare the phenyldiazonium chloride solution (Figure 2), 0.2 mL aniline (C 6H5NH2), 0.35 mL water, and 0.5 mL concentrated hydrochloric acid (HCl) was combined in an Erlenmeyer flask. The mixture was cooled to 4°C in an ice bath, and about 1 mL of cold distilled water was added. A spatula of sodium nitrite was gradually added to the flask, and the mixture temperature kept below 5°C. β-naphthol (Figure 2) was prepared through the dissolution of 0.35 g of the compound in a 4.5 mL 5% sodium hydroxide (NaOH) solution, and the resulting solution cooled to 4°C.
Figure 2. Structure of the reagents phenyl diazonium chloride and β-naphthol involved in the diazo coupling reaction.
After the preparation of the reagents, a piece of cotton fabric was dyed through soaking in the prepared solutions. The fabric was first immersed in the β-naphthol solution for 3 minutes, and dried through filter paper. The dried cloth was submerged in the phenyldiazonium chloride solution for 5 minutes, and then rinsed with running water. The remaining reagents were reacted to produce the Sudan I dye. The phenyldiazonium chloride solution was slowly added to the β-naphthol solution while stirring, and allowed to stand for 3 minutes. The product was filtered and washed with cold water, and the filtrant air-dried. The weight of the resulting dye was measured, and its melting point range determined. III. Results and Discussion
The formation of 1-phenylazo-2-naphthol is performed through a diazo coupling reaction (Figure 3) between phenyldiazonium chloride and the βnaphtholate anion. The coupling reaction involves an attack of the phenyldiazonium ion on the phenol ring of the β-naphtholate anion, forming a ketonic aromatic ring on the naphthol side of the azo intermediate. Resonance stabilisation protonates the oxygen of the ketonic aromatic ring, forming the final product, the Sudan I dye (Christie, 2001).
Figure 3. The diazo coupling reaction between phenyldiazonium chloride and β-naphthol, forming 1-phenylazo-2-naphthol.
During the reaction, two possible intermediates may form, as the phenyldiazonium ion may attack either the ortho sites, as shown in Figure 3, or the para site of the β-naphthol phenolate ring. However, an attack on the ortho sites are more favoured, due to the large steric hindrance by the attached benzene ring on the para position. Also, a comparison of the resonance structures (Figure 4) between the orthoattached and para-attached intermediates show that there is a larger resonance in the ortho position, thus making the said position more stable and thus the more favoured reaction.
Figure 4. A comparison between the resonance structures of the ortho and the para coupling intermediates.
To produce the Sudan I dye, phenyldiazonium chloride was prepared through a three step diazotisation reaction (Figure 5) with aniline in the presence of sodium nitrite and hydrochloric acid. In the first step, nitrous acid dissociates into the
+
nitrosonium ion (NO ) through the protonation of nitrous acid. The nitrosonium ion reacts with aniline in a nucleophilic attack by the arene, to produce an N-nitrosoamine, in the process forming an Nnitrosoaminium ion intermediate. The third step involves the tautomerisation of the N-nitrosoamine to a diazohydroxide, later forming the phenyldiazonium ion through water loss in the presence of hydrochloric acid (Solomons et. al., 2014).
Figure 5. The diazotization reaction for aniline, forming the phenyldiazonium ion, in the presence of sodium nitrite and hydrochloric acid.
The reaction was done in temperatures below 5°C, due to the inherent instability of the reaction product. Arenediazonium salts, as a rule, are only relatively stable at temperatures below 10 °C, as higher temperatures force its decomposition (Figure 6) into the highly stable nitrogen molecule and carbocations. The instability of the salts also provide the molecules needed for the Sandmeyer reactions (Wade, 1987).
Figure 6. The decomposition of phenyldiazonium chloride into nitrogen and a benzene c arbocation.
The reaction mixture was subjected to an acidic environment to generate nitrous acid from sodium nitrite (Wade, 1987). Subjecting the said mixture in a basic solution, on the other hand, achieves the formation of aniline’s conjugate base, with aniline forced to act as an acid in a neutralisation (Figure 7) reaction, where B stands for the base (Vollhardt & Schore, 2011). The resulting conjugate base of aniline may form a salt with the sodium ions from the dissolved sodium nitrite.
Figure 7. The neutralisation reaction between aniline and a base. The preparation of β -naphthol, on the other hand, was done through dissolution of the phenolic compound in a basic environment. This allows the phenol ring of the compound to be converted to a phenolate ion (Ar-O ); this facilitates the coupling reaction, due to its higher solubility in water as compared to its unionised form. In addition to this, the more electron releasing character of the O group as compared to the hydroxyl group more strongly activates the system towards electrophilic substitution (Christie, 2001). The dissolution of β naphthol on acidic solution, however, may be subjected to sulfonation, if sulphuric acid was used in the acidification of the solution.
While the mixing of the reagents for the coupling reaction was done in a single container, two pot synthesis was necessary for the formation of the reaction reagents. This is caused by the contrasting pH requirements for the preparation of the said reagents, as mentioned earlier. The adverse effects of preparing the reagents in a single container, or one pot synthesis, also necessitate the performance of the two pot synthesis for the diazo coupling reaction. The ingrain process was used in dyeing the cotton fabric, due to the characteristically weak H-bonding of the dye molecules’ polar groups to cotton’s hydroxyl groups. This caused by the lack of highly electronegative groups in the structure of cotton (Figure 8), and thus the lack of polar sites that Sudan I can bind to (Landgrebe, 1993). The colouration of the cotton fabric after the process was dark red orange. The dye formed is of low quality, deviating from the normal dark red colour of the pure dye. The levelness of the dye is low, with some parts of the fabric having a darker colouration than others; the dye fastness is relatively good, with the dye not washing away after rinsing the fabric with water.
Figure 8. A three-ring fragment of the polysaccharide cellulose. The colour of the dye was similar to that of the cotton fabric. 2.97 g of Sudan I, equivalent to 11.96 mmol, was produced from the reaction between 0.35 g (2.43 mmol) β -naphthol and 2.19 mmol phenyldiazonium chloride. The theoretical product was computed to be 0.60 g of the dye, putting the reaction at a percent yield of 495%. The very high percent yield may be explained by the presence of impurities in the final product. This is confirmed by the boiling point range of the product, at 3 °C (129-132 °C); in comparison with the theoretical boiling point of the Sudan I (134 °C), the experimental boiling point has a 1.49-3.73% error. The impurities may be traced to the instability of arenediazonium salts at temperatures above 5 °C, as stated earlier; periods when the phenyldiazonium chloride solution was withdrawn from its water bath likely contributed to this. IV. Conclusion The study shows that azo compounds are produced from an electrophilic aromatic substitution reaction between a phenol and a diazonium salt, called the diazo coupling reaction. The diazonium salt may be prepared through the nucleophilic attack of an amine on the nitrosonium ion formed from the dissociation of nitrous acid, through the diazotisation reaction.
The purity of the final product may be improved through purification of the crude azo dye, either through recrystallization or sublimation. V. References Christie, R.M. (2001). Colour chemistry . Cambridge, UK: Royal Society of Chemistry. Landgrebe, J.A. (1993). Theory and practice in the organic laboratory: with microscale and standard scale experiments (4th ed.). Belmont, CA: Brooks/Cole. Marshall Cavendish Corporation. (2003). How it works: science and technology, volume 5 (3rd ed.). Tarrytown, NY: Marshall Cavendish. McMurry, J. (2008). Organic chemistry (7th ed.). Belmont, CA: Thomson Learning.
Seager, S.L., & Slabaugh, M.R. (2013). Safety-scale laboratory experiments for chemistry for today (8th ed.). Belmont, CA: Brooks/Cole. Solomons, G., Fryhle, C., & Snyder, S. (2014). Organic chemistry (11th ed.). Hoboken, NJ: John Wiley & Sons. Stiborova, M., Martinek, V., Rydlova, H. Hodek, P., & Frei, E. (2002). Sudan I is a potential carcinogen for humans: evidence for its metabolic activation and detoxication by human recombinant cytochrome P450 1A1 and liver microsomes. Cancer Research, 62(5678). Vollhardt, P., & Schore, N. (2011). Organic chemistry: structure and function (6th ed.). NY: W.H. Freeman & Co. Wade, L.G. Jr. (1987). Organic chemistry . Englewood Cliffs, NJ: Prentice Hall. VI. Appendix Bnap = β-naphthol S1 = 1-phenylazo-2-naphthol
Theoretical Sudan 1 yield: Percent yield:
Temperature % error: | | | |